For more than a decade, drilling tools as well as milling tools have been developed, e.g., in the form of shank-end mills, that, contrary to integral solid tools, are composed of two parts, viz. a basic body and a head being detachably connected with the same and thereby being replaceable, and in which head the requisite cutting edges are included. In such a way, the major part of the tool can be manufactured from a comparatively inexpensive material having a moderate modulus of elasticity, such as steel, while a smaller part, viz. the head, can be manufactured from a harder and more expensive material, such as cemented carbide, cermet, ceramics and the like, which gives the cutting edges a good chip-removing capacity, a good machining precision and a long service life. In other words, the head forms a wear part that can be discarded after wear-out, while the basic body can be re-used several times (e.g., 10 to 20 replacements). A now recognized denomination of such cutting edge-carrying heads is “loose tops”, which henceforth will be used in this document.
On rotatable tools of the loose top type, a plurality of requirements are put, one of which is that the loose top should be held centered in an exact and reliable way in relation to the basic body. Accordingly, each unintentional eccentricity between the center axis of the loose top and the center axis of the basic body should not be more than 0.01 mm. Most preferably, it should be smaller than 0.005 mm unless exact centricity can be achieved. Another requirement or desire from the users' side is that the loose top should be mountable and dismountable in a rapid and convenient way without the basic body necessarily having to be removed from the driving machine.
Drilling tools as well as milling tools (shank-end mills) of the loose top type are widely known and may be divided into a number of different categories depending on the ideas on which the designs are based. Accordingly, certain tools use loose tops having rear center pins that entirely or partly (together with other coupling details) fulfill the purpose of centering the loose top in relation to the basic body. To this category of tools belongs among others U.S. Pat. No. 6,012,881, which discloses a loose top drill in which a rear coupling part on the loose top is inserted axially in a jaw between two non-compliant drivers, the insides of which include axially running, torque-transferring ridges that engage corresponding chutes in the coupling part of the loose top, besides which a centric pin protruding rearward from the loose top is inserted in a center hole mouthing in the bottom of the jaw. With the pin, a screw mounted in a radial hole in the basic body co-operates that has the purpose of locking the loose top in relation to the basic body. In that connection, the center pin is cylindrical and insertable at a close (form fitting) fit in a likewise cylindrical center hole, in order to, together with concave and convex contact surfaces of the drivers and the coupling part, respectively, fulfill the purpose of centering the loose top. The concurrent requirements of fit not only between the center pin and the hole, but also between the drivers and the coupling part, impose extreme, not to say practically unattainable, requirements of manufacturing precision, and if high accuracy peradventure would be achieved, the mounting and dismounting of the loose top will become particularly cumbersome.
The present invention aims at obviating the above-mentioned disadvantages of the tool according to U.S. Pat. No. 6,012,881 and at providing an improved loose-top tool. Therefore, an object of the invention is to provide a loose-top tool of the type initially mentioned, in which the loose top can be centered in a meticulously accurate way in relation to the basic body, as well as be mounted and dismounted in a simple and smooth way.
Another object of the invention is to provide a tool having a loose top, the centering pin of which can be utilized not only for the exact centering, but also to directly or indirectly—in co-operation with the radial screw—provide a simple and smooth axial locking of the loose top. More precisely, the loose top should be able to resist such negative axial forces that aim to pull the loose top axially out of the jaw, above all in connection with a drill being pulled out of a drilled hole.